Reduction of Electromagnetic Interference in Integrated Circuit Device Packages

ABSTRACT

EMI radiation in an integrated circuit device package ( 10 ) is reduced or eliminated by the introduction of a magnetic material into the encapsulating medium ( 14 ). The permeance of the magnetic encapsulating medium ( 14 ) affects the inherent series inductance of the lead frame conductors ( 16 ) to thereby reduce electromagnetic interference. Ferrite microbeads ( 30 ) are formed around the lead frame conductors ( 16 ) to contain the magnetic flux ( 32 ) generated by an electrical current signal and to attenuate the effects of mutual inductance.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. application Ser. No. 10/684,072filed Oct. 13, 2003, which a division of U.S. application Ser. No.10/040,256 filed Dec. 31, 2001, which is a division of U.S. applicationSer. No. 09/068,685 filed May 13, 1998, which was the National Stage ofInternational Application No. PCT/US 96/17916 filed Nov. 15, 1996,published in English as WO 97/18586 dated May 22, 1997, which claimedthe benefit of U.S. Provisional Application No. 60/006,755 filed Nov.15, 1995.

BACKGROUND OF THE INVENTION

The present invention relates generally to reduction of electromagneticinterference and specifically to reduction of electromagneticinterference generated within an integrated circuit device package.

Advents in the performance of microcomputer based electronics haveresulted in dramatic increases in operating speeds of the logicswitching circuits. Increased switching and operating speeds correspondto increased bandwidths of the electronic signals transmitted within theinterior of an electronic device which become a significant source forelectromagnetic radiation causing interference with the internalcircuitry of the device itself and with other electronic devicesoperating within the vicinity of the device. The electromagneticradiation emitted at these higher frequencies may cause undesirableelectromagnetic coupling between data paths resulting in cross channelinterference.

The amount of internally generated electromagnetic radiation must belimited to the guidelines and regulations set by governmental agenciessuch as the FCC in the United States and CISPR in European countries.Sources of electromagnetic radiation originating externally to thedevice may also affect and interfere with the operation of the device.In general the problems resulting form unwanted electromagneticradiation are classified as electromagnetic interference (EMI).

A recurring observation in the analysis of EMI performance in productsthat use VLSI integrated circuits is that there is a significant amountof emission radiated directly from the integrated circuit package itselfbefore the signal connections from the device are available on anexternal pin. This is particularly evident in devices that have a largenumber of pins, such as a common 208 pin Quad Flat Pack (QFP) device. A208 pin QFP device is typically on the order of 1 inch square with theactual integrated circuit itself occupying only a small amount of thereal estate of the QFP package. Typically, the integrated circuit (IC)is relatively small being on the order of 0.2 square winches to 0.3square inches. As a result, there must be internal conductor leads fromthe IC silicon wafer to the external pins of the device. This istypically implemented with a lead frame of metal strips etched orstamped from a sheet of material to support the integrated circuit chipand to provide a signal path for the input and output (I/O) pins of theQFP device.

In such a design there may be a significant conductor length from the ICitself through the bonding wires and the lead frame conductors to theexternal pins of the device. This is especially true for pins at or nearthe corner of the device, in which case the conductor lead length may bewell over 0.5 inch.

The described physical lead lengths in typical integrated circuitpackaging designs generally cause two problems. The problem is mid andhigh frequency signal degradation introduced by the inherent seriesinductance of the conductor leads which is particularly a problem forthe power and ground feeds. The second problem is that the conductorleads may radiate EMI energy as an antenna thereby interfering with thesignals an adjacent conductor leads in the package and with other signalpaths and components in the electronic device in which the integratedcircuit is utilized. The techniques known in the art for reducingelectromagnetic interference are effective only external to theintegrated circuit device package. Thus, there lies a need for a methodand apparatus to reduce or eliminate electromagnetic radiation internalto the integrated circuit device package itself.

SUMMARY OF THE INVENTION

The present invention provides reduction and elimination ofelectromagnetic radiation in an integrated circuit. The electromagneticradiation is reduced or eliminated, and electrical signals internal tothe integrated circuit package are conditioned internally within theintegrated circuit package itself.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF DRAWINGS

The numerous objects and advantages of the present invention may bebetter understood by those skilled in the art by reference to theaccompanying figures in which:

FIG. 1 is a somewhat diagrammatic representation of the internal designof a typical integrated circuit package;

FIG. 2 is a schematic illustration of the electrical model of a typicalintegrated circuit device package;

FIG. 3 is a somewhat schematic illustration of the electrical model ofan integrated circuit device package utilizing the present invention;

FIG. 4 illustrates the resulting characteristic curve of the electricalsignals conditioned by the present invention;

FIG. 5 is a schematic elevation view of an integrated circuit devicepackage of the present invention illustrating the magnetic flux patternoccurring therein;

FIG. 6 is a somewhat diagrammatic representation of the internal designof an integrated circuit device package utilizing the present invention;

FIG. 7 is a schematic elevation view of an integrated circuit devicepackage of a preferred embodiment of the present invention illustratingthe magnetic flux pattern occurring therein;

FIG. 8 is a somewhat diagrammatic representation of the internal designof an integrated circuit device package utilizing a preferred embodimentof the present invention; and

FIG. 9 is an electrical schematic diagram of the equivalent circuitmodel of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 illustrates the model of a typical Quad Flat Pack (QFP)integrated circuit device. The Quad Flat Pack may comprise a 1.0 inchsquare integrated circuit device package 10. The integrated circuit (IC)12 itself may only comprise a 0.2 inch square or 0.3 inch square waferof silicon containing the actual integrated circuitry of the integratedcircuit package 10. The device package 10 is formed by encapsulating theintegrated circuit 12 in a plastic medium 14 which defines the physicaldimensions of the device package 10. The plastic medium 14 protects andsupports the integrated circuit 12 and contains the lead frameconductors 16 which electrically connect the IC 12 to the externalinput/output (I/O) pins 18 of the device package 10. The lead frameconductors 16 connect to the IC 12 via bonding wires 20 which directlyconnect to strategic circuitry nodes on the IC 12.

FIG. 2 illustrates the electrical model of the typical integratedcircuit package of FIG. 1. FIG. 2 illustrates how the lead frameconductors 16 from the I/O pins 18 to the integrated circuit 12 are notideal transmission lines but in practice exhibit a per length seriesinductance. Longer lengths of the lead frame conductors 16 result inlarger values of the inherent series inductance. Input leads of the leadframe conductors 16 connect the input pins 18 _(IN) of the I/O pins 18to input buffers such as 22 on the integrated circuit 12, and outputleads of the lead frame conductors 16 connect the output pin 18 _(OUT)of the I/O pins 18 to output buffers such as 24 on the integratedcircuit 12. In general, the input and output conductor leads of the leadframe conductors 16 exhibit input series inductance L_(IN) and outputseries inductance L_(OUT).

Ideally, the inductance of the V_(CC) and V_(GND) leads would be zerohenries. In a preferred embodiment of the present invention theeffective inductance of the V_(CC) and V_(GND) leads is effectivelyreduced with multiple parallel branches since there are typicallymultiple V_(CC) and V_(GND) pins in a given integrated circuit devicepackage 10. Utilization of multiple signal paths to reduce the effectiveinductance of a data signal path provided by a lead frame conductor 16is not feasible; therefore the effective series inductances L_(IN) andL_(OUT) of the lead frame conductors 16 preferably exhibit a smallamount of “lossy” inductance. By making the series inductance of leadframe conductors 16 lossy, the detrimental EMI effects of the seriesinductances may be thereby reduced.

Given the construction of the lead frame which provides the lead frameconductors 16, the packaging function for the integrated circuit 12 ispreferably completed by placing the lead frame with bonded integratedcircuit 12 into an injection molding cavity where molten plastic isinjected to encapsulate the lead frame and integrated circuit 12 to formthe device package 10. The plastic material 14 is preferablyelectrically passive and electrically non-conducting so that it willcause no degradation of the electrical signals to and from theintegrated circuit 12.

In a preferred embodiment of the present invention a modeled plasticmaterial 14 having desired electromagnetic properties to advantageouslyaffect the signals to and from the integrated circuit 12 as the signalsare routed through the lead frame conductors 16 of the device isutilized. A small amount of ferrite powder is preferably blended withthe plastic material 14 to achieve the slightly lossy magneticcharacteristic of the encapsulating plastic medium 14 surrounding thelead frame conductors 16. Ferrite is preferred because of its highresistivity and permeability.

FIG. 3 illustrates the effects of the introduction of ferrite powderinto the encapsulating plastic of the integrated circuit device packageof FIG. 1. The introduction of a ferrite material into the encapsulatingplastic 14 alters the permeance of the encapsulating medium 14 andthereby affects the electrical characteristics of the inherent seriesimpedance of the lead frame conductors 16. The ferrite material in theencapsulating medium 14 causes the series inductance of the lead frameconductors 16A and 16B to behave as a lossy inductor L_(F). Further, theferrite material contributes to the mutual inductance M_(F) andresulting coupling primarily associated with adjacent lead frameconductors 16A and 16B. The ferrite material exhibits hysteresis loss,but because ferrite has high characteristic resistivity, it exhibits noeddy-current loss. Increasing the permeance of the physical medium 14surrounding the inductance with the presence of a magnetic material suchas ferrite produces an effect opposite to the effect resulting withmagnetic core inductors; instead of concentrating the magnetic fluxwithin the center of the inductor to thereby augment the effectiveinductance as with a magnetic core, the increased permeance of thesurrounding medium 14 due to the magnetic material tends to distributethe flux throughout the medium away from the inductor therebyattenuating the effective inductance.

FIG. 4 illustrates the resulting preferred characteristic signal shapeof a given data signal when a ferrite material is introduced into theencapsulating medium. The lossy inductor L_(F) as shown in FIG. 3 wouldserve to attenuate only the highest frequency signal components whileintroducing generally little true inductance effects of overshoot andringing associated with the series inductance of the lead frameconductors 16 when no ferrite is present. The Q of the inherent seriesinductance of the lead frame conductors 16 is thereby minimized ratherthan maximized. Thus, the intentionally introduced inductor loss reducesthe undesired effects of the inherent series inductance such asovershoot and ringing.

FIG. 5 illustrates a two conductor mutual coupling model of the presentinvention. Introduction of mutual coupling and signal crosstalk betweentwo adjacent lead frame conductors 16A and 16B would be an undesirableeffect that is preferably minimized. Current flowing into conductor 16Aintroduces magnetic flux 32 through adjacent conductor 16B therebyinducing a current therein. In a preferred embodiment of the presentinvention a relatively small amount of ferrite material is blended inthe encapsulating plastic 14 resulting in a relative permeability of thesurrounding material 14 that is not too high to cause significant mutualcoupling but yet sufficient to desirably affect the series inductance ofthe lead conductors 16. In a preferred embodiment of the presentinvention, the relative permeability of the encapsulating medium 14ranges from 5 to 10.

Regarding the two conductor mutual coupling example as shown in FIG. 3,the actual amount of mutual inductance M_(F) between two adjacent leadframe conductors 16A and 16B is small with respect to theself-inductance L_(F) of each conductor 16. In a preferred embodiment ofthe present invention, the reduction of crosstalk on any particular leadframe conductor 16 may be further achieved by placing that particularlead frame conductor 16 adjacent to a V_(CC) or V_(GND) lead to avoidany coupling to another data signal path.

FIG. 6 illustrates a preferred embodiment of the present invention inwhich mutual coupling between adjacent leads is eliminated. The virtualelimination of the mutual inductance may be achieved by molding thedevice package 10 in two steps. The first step preferably comprisesconstructing the lead frame which provides lead frame conductors 16 andthen forming or molding individual ferrite “microbeads” 30 on each leadframe conductor 16. The microbeads 30 are preferably offset so they donot interfere with adjacent microbeads 30. The microbead 30 areelectrically isolated from the adjacent lead frame conductors 16.

In a preferred embodiment of the present invention the microbeads 30 aremade of pure ferrite material which may be constructed using knownceramic techniques, and the microbeads 30 would be formed as an integralpart of the lead frame 16. The microbeads 30 are utilized in a manneranalogous to the utilization of ferrite bead chokes in radiofrequencytransmission lines and antennas. The bead surrounds the transmissionline and effectively chokes undesired high frequency signals immediatelyexternal to the transmission line that are the source of electromagneticinterference without affecting data signals passing therethrough.

The second step preferably comprises ordinary plastic encapsulation ofthe lead frame that provides lead frame conductors 16 and the integratedcircuit 12 upon completion of the bonding and wiring of the IC 12 to thelead frame. In an alternative embodiment of the present invention theinclusion of a ferrite microbead 30 on any given lead frame conductor 16is optional depending upon the type of signal transmitted thereon. Forexample, V_(CC) and V_(GND) signals perform better if there is noferrite bead 30 on those leads. In a preferred embodiment, the standardlead frame is constructed with a microbead 30 on each lead frameconductor 16. Microbeads 16 may be selectively removed by crushing awaythe undesired beads 30 which is facilitated by the inherent brittlenessof ferrite. Preferably, a simple press may be utilized having smallcrushing pins arranged above the corresponding microbeads to be crushedin which all undesired microbeads 30 may be removed in a single step.

FIG. 7 illustrates a preferred embodiment of the present invention inwhich the magnetic flux is contained within the ferrite beads. Theferrite bead 30 surrounding conductor 16A completely contains themagnetic flux 32 generated by the current flowing into conductor 16A.Thus, no current is induced in conductor 16B from the magnetic flux 32created by the current flowing through conductor 16A. In a preferredembodiment of the present invention, only the microbeads 30 containferrite wherein the encapsulating medium 14 entirely comprisesnonmagnetic plastic. Alternatively, a small amount of ferrite may beblended in with the encapsulating plastic 14 in conjunction with theutilization of ferrite beads to further achieve the reduction ofelectromagnetic interference.

FIG. 8 illustrates the preferred placement of the ferrite beads of thepresent invention relative to the integrated circuit wafer in the devicepackage.

The most effective physical location for the microbeads 30 is as near tothe integrated circuit 12 as possible. With the required close spacingof the lead frame conductors 16 near the IC bonding pads 20, thephysical size of a microbead 30 may not be very large, however theeffects of the reduced size are offset by the fact that the placement ofthe ferrite microbead 30 near the IC 12 is nearly ideal.

FIG. 9 illustrates the resulting electrical circuit model of a givenconductor path in an integrated circuit device package of the presentinvention including an equivalent circuit model 16′ for representing atypical lead frame conductor 16. An output signal V_(OUT) from theintegrated circuit 12 feeds into an output buffer 24 which is externallyconnected through an IC bonding wire pad 20. The bonding wire 20exhibits a small series inductance LB which is small relative to theinductance L_(F) of the ferrite microbead 30. The lead frame conductor16 exhibits a characteristic lumped series inductance L_(OUT) and shuntcapacitance C_(OUT), the effects of which are negligible compared to theinductance L_(F) of the microbead 30, and extends through theencapsulating medium 14. The effects of inductance L_(OUT) andcapacitance C_(OUT) may be further reduced by the blending of ferritewith the encapsulation material 14. The lead frame conductor 16 connectsto an external pin 18 _(OUT) on the exterior of the device package 10.

In view of the above detailed description of a preferred embodiment andmodifications thereof, various other modifications will now becomeapparent to those skilled in the art. The contemplation of the inventionbelow encompasses the disclosed embodiments and all reasonablemodifications and variations without departing from the spirit and scopeof the invention.

1. An integrated circuit comprising: a wafer having circuitry disposedthereon; a plurality of conductors coupled to the wafer; a structurethat encapsulates and supports the wafer; and magnetic material disposedto alter an inductance associated with at least one of the plurality ofconductors.
 2. The integrated circuit of claim 1, wherein the magneticmaterial is at least partially disposed within the structure.
 3. Theintegrated circuit of claim 1, wherein the magnetic material issubstantially homogeneously disposed throughout the structure.
 4. Theintegrated circuit of claim 1, wherein at least a portion of themagnetic material is disposed external to the structure.
 5. Theintegrated circuit of claim 1, wherein the magnetic material comprises aferromagnetic material.
 6. The integrated circuit of claim 1, whereinthe magnetic material comprises a ferrite material.
 7. The integratedcircuit of claim 1, further comprising at least one choke structureformed of the magnetic material, wherein each chock structure associateswith at least one respective conductor of the plurality of conductors.8. The integrated circuit of claim 1, wherein the magnetic materialforms a plurality of choke structures, each of the choke structuresbeing associated with at least one respective conductor of the pluralityof conductors
 9. The integrated circuit of claim 7, wherein thestructure comprises a dielectric material encapsulating at least aportion of the choke structures.
 10. The integrated circuit of claim 7,wherein at least some of the choke structures are disposed external tothe structure.
 11. The method of reducing electromagnetic interferencegenerated within an integrated circuit device package wherein theintegrated circuit device package comprises a wafer having wafercircuitry disposed thereon; a plurality of conductors definingelectrically conductive paths for carrying all of the electric currentflowing to and from the wafer circuitry; and a structure thatencapsulates and supports the wafer; said method comprising applyingmagnetic material that exhibits a lossy characteristic, in the vicinityof at least one of the electrically conductive paths such that themagnetic material is not part of any of the electrically conductivepaths for carrying all of the electric current flowing to and from thewafer circuitry, to attenuate the highest frequency signal componentswhile introducing generally little inductance effects of overshoot andringing associated with the series inductance of the at least one of theelectrically conductive paths.
 12. The method of claim 11, wherein thestructure that encapsulates and supports the wafer comprises anencapsulating medium, and said method further comprises introducingmagnetic material that exhibits a lossy characteristic into theencapsulating medium such that the magnetic material is not part of anyof the electrically conductive paths for carrying all of the electriccurrent flowing to and from the wafer circuitry, so as to cause theseries inductance of the at least one of the electrically conductivepaths to behave as a lossy inductor so as to attenuate the highestfrequency signal components while introducing generally littleinductance effects of overshoot and ringing.
 13. The method of claim 12,wherein a relatively small amount of magnetic material that exhibits alossy characteristic, is introduced into the encapsulating medium suchthat the magnetic material is not part of any of the electricallyconductive paths for carrying all of the electric current flowing to andfrom the wafer circuitry, and so that the relative permeability in thevicinity of the at least one of the electrically conductive paths is notso high as to cause significant mutual coupling with other of theplurality of conductors.
 14. The method of claim 13, where theintroduction of the relatively small amount of magnetic material thatexhibits a lossy characteristic, such that the magnetic material is notpart of any of the electrically conductive paths for carrying all of theelectric current flowing to and from the wafer circuitry, results in arelative permeability of the encapsulating medium that is sufficient todesirably affect the series inductance of the at least one of theelectrically conductive paths.
 15. The method of claim 13, where themutual inductance between the one of the plurality of electricallyconductive paths and an adjacent conductor is small with respect to theself-inductance of each conductor.
 16. The method of claim 11, whereinthe magnetic material that is not part of any of the electricallyconductive paths for carrying all of the electric current flowing to andfrom the wafer circuitry, is applied such that mutual coupling of theone of the plurality of electrically conductive paths and an adjacentconductor is substantially eliminated.
 17. The method of claim 11, wherethe magnetic material that exhibits a lossy characteristic and is notpart of any of the electrically conductive paths for carrying all of theelectric current flowing to and from the wafer circuitry, substantiallysurrounds the one of the plurality of the electrically conductive pathsand effectively chokes undesired high frequency signals immediatelyexternal to the at least one of the electrically conductive pathswithout substantially affecting data signals passing therethrough. 18.The method of claim 17, where the structure that encapsulates andsupports the wafer comprises an encapsulating medium that issubstantially free of magnetic material.
 19. The method of claim 17,where the structure that encapsulates and supports the wafer comprisesan encapsulating medium that contains a small amount of magneticmaterial that is not part of any of the electrically conductive pathsfor carrying all of the electric current flowing to and from the wafercircuitry, to further achieve the reduction of electromagneticinterference.
 20. The method of claim 17, where the magnetic materialthat is not part of any of the electrically conductive paths forcarrying all of the electric current flowing to and from the wafercircuitry, substantially surrounds the one of the plurality ofelectrically conductive paths, at the portion of such one electricallyconductive path relatively near to the wafer.